[G]Ghost/Marauder/Helion - The key to TvZ?

It does not matter what opening you would like to play. As long as it grants a fast expansion and is relatively safe, you are good to go. Try to pressure your opponent while getting an expansion on your own. Once your expansion is done, you should start working on your Ghosts, so we are going to get a Ghost Academy now.

Get as many Ghosts as you can, but don’t waste production time by waiting for another 100 Gas. If you can’t afford a Ghost, get a Marauder instead. Just make sure to have about 5 Ghosts at the 11 Minute Mark in case of possible Muta-play (I will describe this later on in more detail).
Helions should be built until you have about 10 of them. You really don’t need more, because, once in big enough numbers, Ghosts are a pretty good counter to Massive Amounts of Lings. If you loose some, while harassing, rebuild them.
A great threat to this composition is a mid-game roach attack. In order to survive such an attack, build 2-3 bunkers at your natural expansion and repair them if needed.
You should start building your third if you have about 5-6 Ghosts (that is at the 11 Minute Mark, as stated above). If you like, you can do the first push. If you don’t, keep pressuring your opponent with Helions and Dropships. Also, spot his third base as soon as you can and try to deny it (a drop with 4 Marauders is great to do so).
Start both of your infantry upgrades at this time!

Usually Zerg becomes really really strong in the late stages of the game, because of Broodlords, Infestors and Ultralisks (maybe in Patch 1.4, as their build time might be reduced).
Unfortunately for Zerg players, the Ghost counters all of these units. If your opponent is at Hive tech, you should at least have about 20 Ghosts, most of them with full energy, which leaves you with almost infinite sniper rounds to quickly kill Broodlords/Infestors and Ultralisks. Let us take a closer look at this late-game scenario.
You have 20 Ghosts. Each of them can snipe about 8 times, assuming they all have full energy. That would result in 240 snipes. Each snipe does 45 damage. That would mean that we could quickly deal about 10000 damage.
The Marauders are there to back your Ghosts up and your Helions will quickly take out Lings and Broodlings.
If you attack a Zerg, make sure to first focus on Mutalisks or Broodlords with your Ghosts. Only your Ghosts are able to kill Air units, so these are the most precious targets!
Taking a lot of Bases is pretty easy with this tactic, as you will be pressuring the Zerg quite a lot. Just make sure to have enough Barracks at home, to quickly reproduce Ghosts and Marauders (although you should really keep an eye on your Ghosts and make sure they don’t die unnecessarily).

Battlecruisers and Thors are great against Zerg, so it is good to add them to your Ghosts once you are on 4 or more bases. If you decide to do so, you should halt all Helion production. Harassment is not as important anymore as destroying the Zerg’s many bases, so you should focus on units that have a high damage output.

Mutalisks are pretty common in TvT, to secure Mapcontrol for the Zerg player and harass undefended mineral lines. If you don’t react to Mutalisks accordingly, you might loose a fair amount of SCVs, which would set you far behind against a Zerg player that is usually already ahead in workers!
Usually you see the first Mutalisks at the 10:30 to 11 minute mark. In order to make his Mutalisks almost useless, you should start building two Engineering Bays at the 9 to 10 Minute Mark and build one or two turrets at each mineral line once you are able to do so!
Also, you would like to have a couple of Ghosts (about 5 or 6) when the Mutalisks hit, in order to secure your production facilities and further expansions. Use the Snipe ability to quickly get rid of Mutalisks (3 Snipes per Mutalisk are sufficient, so you should be able to kill 2 Mutas every time he makes a harassment attempt, assuming that you have 6 Ghosts).
Once Mutalisks hit a critical number (around the 15 minute mark, assuming he did not neglect upgrades for his ground troops), you should start building some Thors from two Factories! Do not overproduce Thors, 3 to 4 should be more than enough to punish Zergs who try to counter this strategy with massive amounts of Mutalisks (which is, honestly, a pretty good counter, as described in this post: http://www.teamliquid.net/forum/viewmessage.php?topic_id=259174#13

Burrowed Roaches are rather rarely seen and don’t really provide a great threat to this unit composition. However, the Roaches can burrow, making them invisible for you and making them regenerate health rapidly. In order to compensate for that, place a turret at your natural expansion and start getting a Raven to detect Roaches underground. You should swap your Starport with one of your Barrack’s tech-labs to buy some time and be more resource-efficient!

The Baneling is probably the unit that is the hardest to deal with. They are great against Helions and ok against clumped Marauders and Ghosts, once in big enough numbers. Sniping them is difficult, due to their small unit model.
You should still try to keep your Marauders in the front when engaging Banelings. Spread them out a little, to negate the Baneling’s splash damage. The Helions will quickly clean up Zerglings and your Ghosts should try to snipe as many Banelings as possible.

Hehe, good one :p.
*EMP them and laugh at the Zerg!*

Everyone knows that more and more Zergs use the Infestor as their core unit. They use them to harass (fungal on the mineral line), to achieve map control and to win fights. Why can’t Terrans do the same with the Ghost?, I asked myself. Well…they could, but no one seems to do this yet.
Harassment: Harassing with Ghosts is probably the easiest and most fun part when using them. The cloak ability will be our best friend. Just take 2 to 3 Ghosts, cloak them and attack a mineral line. As they do bonus damage to light, they already two-shot Drones, but their snipe ability is faster and more efficient. 3 Ghosts with full energy would be able to kill up to 18 Drones in no time (each Ghost looses about 50 energy because of the cloaking, therefore each has 6 snipes left), this is far superior than Infestors using fungal growth! 18 Drones dead results in 18 Larva the Zerg has to spend on stuff he usually does not want to. Also 18 Drones cost 900(!) minerals.
Map control: Ghosts don’t really provide as much map control as Infestors do. However, building them grants the same advantage. Detection! EMP detects cloaked units, but unfortunately NOT burrowed units. A Raven is still needed against Zerg and can be a great addition to this composition, when fighting a lot of Mutalisks (the PDD blocks Mutalisk shots, minimizing their threat!
Winning battles: Killing units wins a battle and maybe the game. Ghosts are good at killing units, because of their snipe ability, so we build them ! They make enemy casters useless, can deal enormous damage and don’t take bonus damage from anything, as they are no armored, nor light unit (they are biological and psyonic).

The Snipe-ability is of course essential when using this type of Ghost-heavy strategy. I am aware that a lot of players have issues using the Snipe ability in quick succession, so I will show you the most efficient way to do so and provide you useful tips when engaging!
There is basically only one way to use the Snipe-ability effectively.
When engaging, you keep the Snipe Hotkey (R) pressed and left-click the unit you would like to attack a couple of times. You keep doing this until the engagement is over, or the key Zerg units (Mutalisks, Banelings, Broodlords…) are dead. Wasting Snipes on units like Zerglings and Roaches is usually not a good idea!
Another way to use the Snipe ability is to put all of your Ghosts on “Hold Fire” and “Hold position”. This way, you queue Snipe commands with the Shift key, as no attack or move commands are executed when the user actually wants the Ghost to use the Sniper round. However, I do not recommend this version, as your Ghosts won’t attack with their normal attack – less damage is dealt in the end!

I will show you how many snipes you need to kill each Zerg unit (hint: Snipe ignores the target’s armor). Also I will indicate if the unit is a good target to use the Snipe ability on or not!

- Zergling: 1 Snipe; Sniping is not advised at all!
- Baneling: 1 Snipe; Sniping is advised
- Roach: 4 Snipes; Sniping is not advised
- Hydralisk: 2 Snipes; Sniping is (sometimes) advised
- Mutalisk: 3 Snipes; Sniping is advised
- Corrupter: 5 Snipes; Sniping is not advised
- Broodlord: 6 Snipes; Sniping is advised
- Ultralisk: 13 Snipes; Sniping is advised
- Infestor: 2 Snipes; Sniping is advised
- Drones: 1 Snipe; Sniping is (sometimes) advised